This invention relates to a novel biomaterial which does not exist in the present medical field, namely a biodegradable polymeric clayey and sticky substance which has such properties that it shows tackiness, plasticity and shape holding ability at a temperature of from skin temperature to body temperature in human (approximately from 30 to 40xc2x0 C.) and can give optional shapes at body temperature or more by increasing its fluidity, and to materials which use this novel polymeric clayey and sticky, such as a hemostatic material, an adhesive material for tissues, a prosthetic material for tissue reconstruction use, a carrier of drug delivery system (DDS), a plugging material, an accretion-preventing in material and a scaffold material for tissue engineering.
Bioresorbable materials have been studied markedly actively in recent years and put into practical or trial use not only as sutures for surgical operation use but also as various osteosynthetic materials, carriers for the drug delivery system and scaffolds, fillers and prostheses for living body reconstruction in the tissue engineering.
Typical examples of polymers to be used for these many purposes include a wide variety of substances for various uses, such as polyglycolic acid (PGA), polylactic acid (PLA), PLA/PGA copolymer, polydioxanone (PDS), poly-xcex5-caprolactone (PCL), polytrimethylene carbonate (PTMC), poly-D,L-lactide (P-D,LLA) and other copolymers produced by using monomers which constitute these polymers.
As such polymers, substances having various molecular weights, ranging from a low molecular weight of several hundreds to a high molecular weight of exceeding one million, have been synthesized and examined for their suitability for respective uses. For example, amorphous polymers having low molecular weight have been tried for certain uses which do not require strength, such as carriers of DDS and scaffolds of living body reconstruction, and crystalline polymers having relatively large molecular weight have been put into practical use as sutures and osteosynthetic materials which require strength.
These materials, however, are limited to certain shapes such as fine powders, granules, films, sheets, porous bodies, fibers, filaments (threads), rods, plates and screws. These shapes can be changed only to such a degree that each material can be used by cutting it into a certain shape suited for its use, so that these materials cannot be used by changing them into desired shapes just before their use by optionally forming them into three-dimensional directions.
However, certain materials which are implanted into the living body in the action of surgical operation, such as hemostatic materials, adhesives, plugging materials, accretion-preventing materials, prosthetic and scaffold materials for use in the living body reconstruction and carriers of DDS, are ideal as biomaterials from the handling and functional points of view, with the proviso that each of them can be used by changing it easily into an optional shape fitted to the operating region when surgical operation is carried out and that, during the process of the resorption (i.e., degradation and absorption) of the material in the living body and its final excretion from the living body, tissues can gradually penetrate into a region where the material is indwelt and replace the region so that the shape and tissues can be reconstructed and restored into the original conditions. Unfortunately, however, virtually nothing is known so far about biomaterials which satisfy such requirements.
The following describes each of known cases.
[Hemostatic materials]
Among known hemostatic materials, bone wax conventionally used as a hemostatic material for stopping bleeding from the bone marrow is a product obtained using a natural material, bees wax, as the main component and mixing it with isopropyl palmitate and salicylic acid. This hemostatic material is frequently used in certain fields such as orthopedic surgery and plastic surgery where treatment of bones is carried out.
However, since this has poor biocompatibility and affinity and is not biodegradable, it remains as a foreign substance in the living body for a prolonged period of time and migrates inside the living body in some cases, so that this material may be excreted through the skin when used in a superficial disease close to the skin or it may induce infection or inflammation due to foreign body reactions in the living body.
[Adhesive materials]
The adhesive materials for medical use are divided into those which adhere (A) soft tissues and those which adhere (B) hard tissues (such as bones and teeth).
The materials (A) which adhere soft tissues include 1) cyanoacrylate system, (2) fibrin paste, (3) gelatin paste and (4) polyurethane system. However, the material (1) stimulates the living body when hardened and poses a problem in terms of the metabolism of degraded products. Also, the material (2) has low adhesive strength and has an immunological problem, and the material (3) also has low adhesive strength and toxicity of its cross-linking agent, formaldehyde or glutaraldehyde, causes a problem. In the case of the material (4), isocyanate as its starting material poses a problem in terms of its safety upon the living body.
On the other hand, among the materials (B) which adhere hard tissues (such as bones and teeth), bone cement is frequently used in which polymethyl methacrylate (PMMA) monomer (MMA) is mixed with powder of bioactive ceramics such as hydroxyapatite and peripheral hard tissues are adhered and repaired at the time of the polymerization curing of the mixture. This material, however, has disadvantages in that it generates considerably high temperature at the time of polymerization and damages the peripheral tissues by the heat, the monomer remained after the polymerization causes damage upon the living body due to its toxicity, and the poor toughness of the cement causes prolonged cement fracture or its delamination from the tissues.
The fibrin paste which is regarded as most useful material among the adhesive materials for use in soft tissues is a biomaterial originated from the living body and artificially uses the tissue conglutination reaction of fibrin. That is, this is a living body system adhesive material which uses its wound-conglutinating reaction in which a water-soluble plasma protein, fibrinogen, is selectively hydrolyzed into fibrin by the enzymatic action of thrombin and the conglutination is effected by the gelling of fibrin through its molecular association.
It has been tried to use this material as a substitute of sutures for the purpose of carrying out anastomosis of peripheral nerves and capillary vessels or in the field of blood vessel surgery and cranial nerve surgery for the purpose of reinforcing the operating area. It has been tried also in plastic surgery for the purpose of carrying out bone connection and in blood stanching and skin graft fixation of burn patients.
This fibrin paste has many advantages, for example, 1) it is a physiological function-applied adhesive material, 2) it is not related to platelets and in coagulopathy, 3) its adhesion is relatively quick, 4) it does not require excess heat and pressure, 5) it is not affected by moisture in the adhesion region and 6) it is appropriately absorbed due to it high affinity for tissues, but it also has fatal drawbacks in that its adhesion is weak and, being a blood preparation, it has a probability of causing viral infection. In addition, though it is known that this material has actions to enhance regeneration of capillary vessels and accelerate ossification, it has been used broadly but only as a hemostatic material.
Application of the adhesion by hardening of gelatin (collagen) to regions where not so strong adhesion is required has also been expected but hardly put into practical use in reality because of the toxicity of its hardening agent, foreign body reaction of the hardened gelatin and its insufficient physical characteristics.
[Reconstruction protecting materials, plugging materials, accretion-preventing materials, prosthetic materials, fillers, scaffolds for reconstruction use and carriers for drug delivery use]
Since it is desirable that these biomaterials are present temporarily in the living body for a temporal assistance of therapeutic treatments and then finally absorbed and excreted from the living body, it is desirable that raw materials which constitute these materials are bioresorbable as a general rule.
As an example of the reconstruction protecting materials, membranes for use in the treatment of periodontal diseases and reconnection of peripheral nerves have been examined. The object of these membranes is to secure places where tissues are regenerated, while positively assisting repair and reconstruction of tissues by keeping routes for supplying nutrients and cytokines as drugs into the tissues. Examples of the protecting membranes for reconstruction use include a non-absorbable micro-porous teflon (Goatics (registered trademark)) and absorbable materials such as poly-L-lactic acid, a copolymer of L-lactic acid with glycolic acid and a copolymer of L-lactic acid with e-caprolactone.
Similar porous and non-porous films or sheets have also been examined as accretion-preventing membranes.
Examples of the fillers so far examined include a membrane of low molecular weight poly-L-lactic acid, poly-D,L-lactic acid, a copolymer of D,L-lactic acid with xcex5-caprolactone or a copolymer of glycolic acid with xcex5-caprolactone, alone, or its powder or heteromorphic form, or powder or granules of hydroxyapatite or xcex1- or xcex2-tricalcium phosphate or its mixtures with the just described polymers.
As the carriers for use in the drug delivery system, certain materials such as non-porous or porous films, sheets (plates) and granules (powders) of the aforementioned copolymers have been examined.
Since the parts to be adhered are living bodies as has been described in the foregoing, the medical adhesive materials to be used in the living body require difficult conditions other than those for industrial purposes (particularly a condition that they are safe upon the living body), and it is not so easy to satisfy these conditions. Among clinical application cases of the aforementioned fibrin paste, there is a case which does not essentially require high adhesive strength. That is, it is used for the purpose of effecting temporal fixing so that spontaneous connection is completed thereafter by self-repair. In order to effect self-repair of damaged tissues, the adhesive material should not inhibit contact between newly formed tissues by remaining on the region to be adhered and connected. Also, this adhesive material must be a biocompatible material having no toxicity and injurious property as a matter of course. In addition, it is necessary that this material can be sterilized by a certain method.
The materials to be used as plugging materials, accretion-preventing materials, prosthetic materials or fillers will become more ideal biomaterials when they can simultaneously give a shape-reproducing function to restore the original shape of the damaged region and a positive chance for its treatment, in addition to their function to exist in the repairing and reconstructing region of the living body merely as a temporal plugging having a certain shape. That is, they will become more ideal biomaterials if they can be formed into the three-dimensional shape of the region of living body to be repaired, at will at the time of surgical operation, so that they can adhere to the peripheral tissues at the surface, and also if they can restore the living body both morphologically and functionally, by effecting transfer, induction and penetration of the peripheral tissues into the region during the process of their gradual degradable and absorption in the living body and their final excretion from the living body.
However, no biomaterial has been developed which has necessary characteristics for satisfying the just described object, namely (1) its fluidization and plastic deformation can be effected at a temperature not so higher than the body temperature so that it can be easily handled by surgions, (2) it shows tackiness with a living body heat, (3) it can be formulated and mixed intraoperatively without denaturing bio-substances or drugs such as bone fragments and hormones by heating high temperature and (4) the material is bioresorbable by itself and disappears at the relatively early stage after the treatment.
With the aim of creating a material having the aforementioned properties and providing it to the practical field, the inventors of the present invention have conducted intensive studies and achieved this object by not using a biomaterial having a probability of causing antigen-antibody reaction but by preparing and synthesizing a hydrolyzing type synthetic degradable and bioabsorbable polymer which undergoes enzyme-non-specific biodegradation.
That is, the bioresorbable clayey and sticky substance of the present invention which resolves the aforementioned problems is a copolymer comprising two or more bioresorbable monomers, wherein they show tackiness, plasticity and shape holding ability at a temperature of from skin temperature to body temperature in human (approximately from 30 to 40xc2x0 C.) and can give optional shapes at a temperature which is higher than the body temperature but not so high temperature by further increasing its fluidity.
Other objects and advantages of the present invention will be made apparent as the description progresses.